LiDAR Captures Accurate As-Built Condition of Combined Sewer Overflow Pipes in City of Milwaukee

By Jon Chapman and Robert Seleen, P.E.

The City of Milwaukee Department of Public Works (DPW) wanted to better understand the existing condition of their next stretch of combined sewer overflow (CSO) pipe to be slip lined. They were interested in finding a cost-effective and timely solution that would mitigate the construction delays and change orders that they had experienced on prior projects. The City contracted with the civil engineering and surveying firm of raSmith (Brookfield, Wis.) to provide advanced survey technologies and create current and very accurate as-built documentation of their CSO pipes.

The City’s Combined Sewershed 3016 is comprised of over 4,000 acres and incorporates a large Combined Sewer Overflow (CSO) network. Pipes range in size between 66 and 144 inches in diameter and are generally located 35 to 90 feet underground. Installed in the 1880s to 1920s (with additional installations in the 1990s), the aging system requires regular rehabilitation.

Since the 1960s the City of Milwaukee DPW has identified and repaired pipes within the network. Past repairs have involved grout work, formed concrete liners, steel liners, shotcrete, and fiberglass mortar pipe (slip lining). Slip lining is a trenchless technology that has been used since the 1940s to repair leaks and rehabilitate existing pipes and is generally considered to be very cost effective. Slip lining is completed by installing a smaller, carrier pipe into a larger existing host pipe.

The particular section of pipe that the DPW was recently looking to slip line was comprised of approximately 3,700 linear feet of pipe, ranging in diameter between 6 and 7.5 feet, at depths ranging between 35 and 50 feet. This stretch contained three radii and four tangent sections of pipe. While there are several different methods used to install a slip line, this particular project utilized what is known as the segmental method. This construction process involves excavating and exposing a section of existing pipe, allowing a rigid slip liner to be lowered into the pipe with a crane. Nominal length sections of 15 to 20 feet of slip liner are lowered one at a time into the pipe. Each segment then rides on a series of rails that are preinstalled in the bottom half of the pipe. After each pipe segment is lowered into the opening and is resting on the rails, it is then pushed horizontally into place by a skid loader that is lowered in before the pipe segments. Once each segment is pushed into the correct horizontal position, the annular space between the two pipes is grouted and the ends are sealed.   

One of the challenges with this process presents itself when trying to slip line a curved section of pipe. Because each segment of slip liner is rigid and straight, and is pushed horizontally into place, it is very important that the length of each segment is no longer than what the radius of the existing pipe curve will allow to be pushed through it to avoid being hung up or stuck. Assuming you have confidence in the accuracy of the existing drawings, these maximum lengths can theoretically be calculated. Accuracy issues with old drawings aside, existing drawings are not going to show certain as-built conditions like pipe deflections caused by settlement or shifting, sediment buildup or other deposits that may also impact the ability to slide the slip liners through the pipe.

Having faced the afore-mentioned challenges on prior slip lining projects, which inherently resulted in multiple construction delays and change orders, the City of Milwaukee DPW began working with raSmith who utilized static LiDAR coupled with conventional survey control and tools to create current and very accurate as-built documentation of the pipes. The City required a series of deliverables including an alignment and profile of the existing CSO invert, 2D pipe sections at 100-foot stations in the tangent sections of the pipe, and 25-foot stations in the curved sections of the pipe. Additional sections were also required at any observed deflection or deformation points.

raSmith tackled the project by first establishing tight survey control at ground level. This included establishing a series of control points along the corridor and near the various manhole access points. Survey control points were also established in the bottom of the pipes at the manholes by inverting a static LiDAR scanner over the manhole and conducting a series of inverted scans that captured control points both above ground and down in the pipe. Additional below ground control points were then established between the manhole access points, and basic stationing was established.

raSmith then used a static LiDAR scanner to scan the inside of the pipe throughout the entire 3,700-foot length to create a 3D point cloud of the interior conditions. During the field work data collection period, there was also approximately 6 to 8 inches of flow (primarily ground water infiltration) in the pipe, which prevented the capture of reliable point cloud data below the water surface, obscuring the pipe invert as well as some of the sediment buildup and deposits. raSmith utilized a robotic total station to map those features with a conventional survey approach to supplement the point cloud data.

This approach allowed raSmith’s office technicians to not only create the alignment and plan and profile documents, but also specifically map the areas with sediment buildup and wandering deposits, which may also potentially hinder the slip lining construction process. Furthermore, the series of cross sections clearly illustrated the existing profile of the pipe compared to a perfectly round pipe of the same size and alignment. As a value-added deliverable, raSmith also provided the City of Milwaukee with the 3D point cloud data and a 3D surface of the inside surface of the pipe.

All of the resulting documentation was included in the bid document package sent to general contractors to assist them with properly planning for and bidding on the subsequent construction work. Of the general contractors that utilized this information in their planning and budgeting efforts, their bids inherently were very similar and considerably lower than those general contractors who did not utilize that same information.

The DPW issued a contract in November 2019 to slip line three of the tangent sections and two of the radii that were surveyed using static LiDAR; the other tangent and radius will be rehabbed in a future DPW contract. The general contractor that was awarded the construction bid used the provided horizontal alignment information to work with a pipe supplier to best determine the work shaft locations and length of pipe that could be slipped into place from each location. One work shaft was located downstream of a radius that enabled the contractor to slip pipe in both directions. In the tangent direction full 20-foot pipes were used. In the direction of the radius the contractor chose to use 10-foot sections, which allowed them to push the pipe through one radius, through a tangent section and then through the second radius. A second work shaft was located upstream of this radius where the contractor was able to again use 20-foot pipe sections to connect to the 10-foot section at the end of the radius.

Having access to cross sections and a 3D surface of the existing pipe allowed the DPW’s engineers to quickly review the contractor’s shop drawings and proposed slip lining procedure while providing staff with a sense of confidence that no surprises would be encountered during construction. This project was a great success for the City of Milwaukee DPW because of the general contractor’s methods, work shaft location and pipe length selection. The project’s schedule and budget were based on previous slip lining work contracted by the City, and the work was completed in roughly half the time scheduled and under budget.

Jon Chapman, senior project manager at raSmith, served as the project manager for the City of Milwaukee Combined Sewer Overflow (CSO) project. He has more than 27 years of experience in LiDAR (3D laser scanning), UAS, and 3D visualization services.  Jon is also an FAA Certified UAS Pilot. raSmith is a multi-disciplinary consulting firm comprising civil engineers, structural engineers, land surveyors, development managers, landscape architects, and ecologists.
Robert Seleen is a floodplain hazard mitigation manager in the environmental engineering section at City of Milwaukee Dept. of Public Works. He has worked in the Milwaukee area for over 15 years as a laborer, surveyor, inspector, and professional engineer for private companies and local government. His dedication to water quality improvement projects is based on his desire to ensure future generations can enjoy Wisconsin and all of its beauty.